Many electrical products have a display device, and many different parameters are used to determine quality of the display devices, where contrast ratio (CR) is one important parameter. As the CR increases, an image of the display device becomes more vivid and more colorful. A common display device divides grey scales into 255 grades, where the CR is equal to a specific value between surface luminance of 255th grey scale L255 and surface luminance of original grey scale L0. An equation of the Cr is: CR=surface luminance of L255/surface luminance of L0.
A surface luminance of the 255th grey scale L255 of a thin film transistor liquid crystal display (TFT-LCD) device is not prone to increase because of limiting factors, such as a maximum backlight brightness and penetration rate of glass. A surface luminance of original grey scale L0 of the TFT-LCD device obtains through a local dimming method.
Currently, two main backlight local dimming methods are as follow:
A: a first backlight local dimming method is a side-light backlight local dimming method. An advantage of the first backlight local dimming method is acceptable cost, and a disadvantage of the first backlight local dimming method is few backlight blocks of the light units, a number of the backlight blocks of the light units is generally 2*6 or 2*8.
B: a second backlight local dimming method is direct-light backlight local dimming method. An advantage of the second backlight local dimming method is a plurality of the backlight blocks of the light units and good dimming effect, and a disadvantage of the second backlight local dimming method is that cost and the number of the backlight blocks of the light units are directly proportional.
The number of the backlight blocks of the light units is generally determined by a number of dimming channels of a backlight driving unit, where the backlight driving unit drives a light emitting diode (LED) light bar to light. Taking a backlight having 16 backlight blocks of the light units for example, a backlight driving unit having 16 dimming channels is needed to drive the LED light bar of each of the 16 backlight blocks of the light units to light, or the LED light bar of each of the 16 backlight blocks of the light units is driven to light by a plurality of backlight driving units. For a large size LCD panel, the direct backlight has a plurality of the backlight blocks of light units, and the dimming channel of the backlight driving unit correspondingly increases, thereby increasing costs.